Dragon Slave shouldn't be any Lina exclusive spell anyway. What was her exclusive spell again... Giga Slave? We should get worried if the strange creature attempts to use that as well.

One of the things that bothered me about this show is indeed the seemingly limited amount of spells and the fact just about anyone can use them. Zel also had a super spell in the beginning, Ra Tilt was the name if memory serves, said to almost rival Dragon Slave, but then suddenly also Amelia could use it, despite the fact she was presented as quite a clumsy Defender of Justice. At that point I ceased to think even the bigger magics were anything special in Slayers, except for that one cataclysmic spell of Lina's.